1. Field of the Invention
The present invention relates to an X-ray apparatus, and more particularly, to a technique which is effectively applied to an exposure controller for properly controlling X-ray conditions at the time of radiographic exposure in X-ray diagnosis.
2. Related Background Art
A radiation dose control for properly setting X-ray conditions at the time of radiographic exposure is indispensable when an operation is shifted from fluoroscopic exposure to radiographic exposure or when a series of radiographic exposures are sequentially performed. An X-ray automatic exposure control method in an X-ray apparatus for shifting the operation from the fluoroscopic exposure to the radiographic exposure has been improved over a long time. However, proper setting of the X-ray conditions is difficult due to influences by contrast media, bones, and the like when X-rays are transmitted through the object. The method is strongly requested to be improved, especially in diagnosis of the digestive organs for which the method is often used.
As a conventional X-ray automatic exposure control method, for example, there is an X-ray automatic exposure controller described in Japanese Patent Application Laid-Open No. (JP-A) 57-88698.
The X-ray automatic exposure control apparatus detects partially picked up X-ray transmission images of an object by a plurality of photo-diodes, and radiation time of X-rays upon X-ray radiographic exposure is controlled on the basis of outputs of detectors of selected photo-diodes among the plurality of photo-diodes.
That is, since the X-ray automatic exposure controller can control, in a real time manner, the radiation time of X-rays upon the X-ray radiographic exposure on the basis of the output signals of the photo-diodes, the control of which is not influenced by individual difference of an object, the X-ray radiographic conditions, and the like can be performed.
As another X-ray automatic exposure control method, there is an X-ray diagnostic apparatus described in JP-A-62-15800.
The X-ray diagnostic apparatus obtains average thickness and the maximum and minimum thickness of an object from video signals outputted from an X-ray detector at the time of X-ray fluoroscopic exposure of the object, determines X-ray radiographic conditions so that the contrast of an X-ray radiographic image of the object becomes maximum, and controls the radiographic exposure.
That is, since the X-ray radiographic conditions of the object are determined by using the video signals outputted from the X-ray detector, since it is unnecessary to prepare photo-diodes which are required in the above-mentioned JP-A-57-88698 and the X-ray radiographic conditions can be controlled with a simple apparatus construction.
Generally, when the exposure control is performed by using the video signals, it is necessary to execute a control of radiographic exposure time in time scale shorter than a speed of reading the video signals. Therefore, a real-time control cannot be performed.
As described in the above-mentioned JP-A-62-15800, the X-ray radiographic conditions of the object have to be preliminarily determined.
Meanwhile, in the X-ray apparatus, it is desirable to suppress the dose of X-rays radiated to the object and to obtain a high-quality X-ray image, especially, in the radiographic exposure to a region (aimed field) in which an examiner is interested.
As a conventional X-ray apparatus, there is an X-ray fluoroscope radiographic apparatus in which an X-ray tube and a two-dimensional X-ray detector are used so as to face each other and fluoroscopic two-dimensional X-ray images of an object are obtained from various directions, and the radiographic two-dimensional X-ray images of an object is continuously obtained. There is also an X-ray rotatographic apparatus for continuously acquiring two-dimensional X-ray transmission images of an object while rotating an X-ray tube and a two-dimensional X-ray detector which are arranged so as to face each other around the object. These X-ray apparatus can display images acquired by the radiographic exposure in a real time manner and can continuously display the images after the radiographic exposure.
There is a cone-beam CT having the X-ray rotatographic apparatus as a measurement system and three-dimensional image reconstructing means as an image processing unit. The cone-beam CT can image three-dimensional distribution of absorption coefficients of the object from a series of acquired two-dimensional X-ray transmission images.
There is also an X-ray CT which has a one-dimensional X-ray detector or a detector array comprising a plurality of one-dimensional detectors and acquires one or a plurality of X-ray slice images at once. In the X-ray CT, however, it is necessary to repeatedly execute measurement in order to image the three-dimensional distribution of the absorption coefficients. On the contrary, the cone-beam CT can collect the two-dimensional transmission images for reconstructing a number of X-ray slice images at once and is therefore characterized in that a three-dimensional image can be acquired in shorter time.
For example, such a cone-beam CT is disclosed in "JAMIT Frontier '95" p. 23-28, which uses a detector having an X-ray image intensifier, an optical lens system, and a television camera as a two-dimensional X-ray detector. There is also a cone-beam CT using a detector having a fluorescent screen, an optical lens system, and a television camera as a two-dimensional X-ray detector disclosed in "BME" Vol. 33, special edition (theses of the 34th meeting of Japan society of ME) p. 109.
The two-dimensional X-ray detector has, however, a narrower dynamic range as compared with that of a one-dimensional X-ray detector used for the X-ray CT, so that a minute difference of the dose of X-ray entering the detector cannot be detected by a measurement system. The contrast resolution of measurement data of the two-dimensional X-ray detector is consequently lower than that of the one-dimensional X-ray detector. The contrast resolution of a three-dimensional image obtained by the cone-beam CT is therefore inferior to that obtained by a CT having the one-dimensional X-ray detector. Consequently, in the conventional cone-beam CT, an image cannot be obtained under conditions where absorption of X-rays by an object is large and an X-ray transmitted rate is small, so that there is a drawback that the maximum object thickness for obtaining an image is small.
On the other hand, the radiographic conditions in case of performing the rotatographic exposure by using the two-dimensional X-ray detector can be principally set to the same as those of a general X-ray CT. In the rotatographic exposure using the two-dimensional detector, as a conventional technique for increasing the maximum object thickness for obtaining an image by improving the contrast resolution for the object having small X-ray transmitted rate, it is considered to use automatic exposure control means which is generally used for controlling X-ray dose in X-ray fluoroscopy. That is, when the absorption of X-rays by the object is large, the X-ray dose is automatically increased to improve the level of an image, thereby compensating the narrowness of the dynamic range of the detector.
An X-ray apparatus using the automatic exposure control means is disclosed, for example, in JSRT No. 45, Vol. 8, p 1014. According to the X-ray apparatus, an optical sensor is mounted in an optical lens system constructing a two-dimensional X-ray detector, the average brightness of an arbitrary aimed region on an output fluorescent face of an X-ray image intensifier is measured by the optical sensor, and an X-ray tube voltage is controlled so that an output level of the optical sensor becomes constant.
In case of applying the idea of the automatic exposure control means to a rotatographic apparatus or a cone-beam CT, when the thickness of the object is increased at a predetermined rotation angle and the X-ray absorption of the aimed region is increased, the output level of the optical sensor decreases. By automatically executing a control to increase the X-ray tube voltage in order to compensate the decrease in the output of the optical sensor, the dose of X-ray radiated from the X-ray tube is increased, and as a result, the level of the image is raised. The narrowness of the dynamic range of the two-dimensional X-ray detector can be consequently compensated.
A general X-ray CT in which X-ray dose is changed at each exposure angle in order to reduce the dose of X-rays radiated to an object is described in JP-A-53- 126291. In the X-ray CT, X-ray radiographic exposure is performed to an object as a preliminary measurement. The shape of a slice or information (X-ray absorption information) regarding the X-ray absorption of the object is preliminarily formed on the basis of the acquired X-ray image. In the measurement, by controlling applying time of a voltage which is applied to the X-ray tube on the basis of the slice shape or the X-ray absorption information, the dose of X-rays entering the detector is kept constant.
The inventors of the present invention examined the conventional techniques regarding the automatic exposure control in which operation is shifted from the fluoroscopic exposure to the radiographic exposure and found out the following problems.
In the X-ray automatic exposure control apparatus described in JP-A-57-88698, since a number of photo-diodes are necessary for performing an accurate exposure control, there is a problem that costs are high.
Since a controller for controlling the number of photo-diodes and also the exposure on the basis of a number of inputs is more complicated as the number of photo-diodes increases, there is a problem that the costs further increase.
Further, since the photo-diode and the X-ray detector have different sensitivity characteristics to light, there is a problem that the difference makes an accurate exposure control difficult.
On the other hand, with respect to X-ray scattering which occurs when X-rays are transmitted through an object, it is known that intensity and distribution of the scattered X-ray are changed generally by a tube voltage of an X-ray tube, the kind of an X-ray filter, the thickness of the object, a distance between the object and an input face of the X-ray detector (hereinbelow, referred to as an "air gap"), the kind of an X-ray grid, and the like and the X-ray scattering is also influenced by the size of an X-ray exposing area.
In an apparatus using an X-ray image intensifier (hereinbelow, described as an "X-ray I. I.") as a detector, it is known that veiling glare occurring when an X-ray image is converted into an optical image changes the intensity and distribution of the scattered X-ray in accordance with an I. I. mode which specifies a detection area.
In an apparatus using the X-ray I. I. and a television camera, therefore, in addition to primary X-ray and scattered X-ray entering the X-ray I. I., the result also including the veiling glare occurring in the X-ray I. I. is picked up by the television camera, that is, converted into video signals.
In the X-ray diagnostic apparatus described in JP-A-62-15800, when the exposure control is executed by using the video signals, since influence by the X-ray scattering and the veiling glare is not considered, accurate thickness of the object cannot be obtained. Consequently, there is a problem that X-ray radiographic conditions cannot be accurately determined.
The inventors of the present invention examined the conventional techniques regarding the continuous radiography and found out the following problems.
In the measurement system for rotatography used for the cone-beam CT, in order to improve the S/N ratio of the three-dimensional image of the aimed region for the same X-ray dose sum, the distribution of the X-ray dose and the image level have to be adjusted. The X-ray dose sum denotes a sum of X-ray dose used for the radiographic exposure at every angle in a series of rotatographic exposures.
In each of the rotatographic apparatus, the cone-beam CT, and the X-ray CT described in JP-A-53-126291 each having the automatic exposure control means, when one X-ray image is acquired, it is the goal to acquire the X-ray image having picture quality as high as possible. That is, in the conventional X-ray apparatus, since the distribution of the X-ray dose is not adjusted so that the S/N ratio of the three-dimensional image of the aimed area is proper for the same X-ray dose sum, there is a problem that the contrast resolution of a three-dimensional reconstructed image cannot be further improved. Another problem is that the dose of X-rays radiated to the object cannot be further reduced.
With respect to the fluoroscopic or continuous exposure, in the automatic exposure control having the logic of increasing or decreasing the X-ray dose in correspondence to change with time in the characteristics of the object, there is a problem such that when the X-ray tube voltage changes, the image contrast of the same region of the object varies.
In the measurement system of the X-ray apparatus, the X-ray dose has to be properly set at each exposure angle and the image signal level has to be adjusted in order to maximally use the dynamic range of the detector according to the set X-ray dose. In order to adjust the image signal level, it is desirable to adjust a camera input light level by adjusting an optical iris in the optical system.
A digital X-ray radiographic apparatus having a mechanism of adjusting the camera input light level is described in JP-A-4-336045. According to the digital X-ray radiographic apparatus, the radiation dose of X-rays is controlled by the above-mentioned automatic exposure control. The amount of light entering the television camera is adjusted by providing an iris mechanism in front of the television camera and controlling the iris mechanism. Specifically, the control method adjusts the iris on the basis of the ratio of the maximum level of the video signal as an output of the television camera to a peak level of the video signal. In the digital X-ray radiographic apparatus, however, since the control of the dose of X-rays radiated to the object and the control of the amount of light entering the television camera are separately performed, the dynamic range of an analog to digital (AD) converter for converting the X-ray image into digital signals can be maximally used. However, the problem regarding the automatic exposure control cannot be solved. The invention of the digital X-ray radiographic apparatus is therefore different from the present invention.
When the X-ray tube voltage is changed, the X-ray energy spectrum, that is, X-ray quantum energy distribution is changed. When the X-ray tube voltage increases, the average level of the X-ray quantum energy increases. As a result, the X-ray absorption coefficient of an object region of the object or a background region is generally reduced and the ratio of the X-ray absorption coefficient of the object region to that of the background region is changed. The absorption coefficients obtained by three-dimensional reconstruction are, therefore, inaccurate and vary according to position. Consequently, variance of the image is increased and there is a problem that the contrast resolution deteriorates as compared with a case of controlling the X-ray dose while the tube voltage is constant.